The invention relates to a method and arrangement for establishing a connection between a base station and mobile station. The invention is especially applicable to digital TDMA (Time Division Multiple Access) and CDMA (Code Division Multiple Access) cellular systems which utilize extended cell operating ranges. One such TDMA cellular system is the GSM (Global System for Mobile telecommunications) which uses the 450-MHz frequency band.
In its active state a terminal of a cellular radio system always attempts to select a coverage area, or cell, of a given base station to camp on. Conventionally, the selection of the cell has been based on measuring the strength of a received radio signal either at the terminal or at the base station. In the GSM, for example, each base station transmits a signal on a so-called Broadcast Control Channel (BCCH) which is given different frequencies for adjacent base stations, and the terminals measure the strengths of the BCCH signals received, and decide on the basis of the measured strengths which cell is the most advantageous in regard to the radio link quality. The base stations also transmit to the terminals information about the BCCH frequencies used in the neighboring cells so that the terminals know what frequencies they have to listen to in order to find the BCCH transmissions of the neighboring cells. In each cell the BCCH transmission also includes information about how the terminals can make so-called random access requests in that particular cell in order to establish connections for calls. The base stations use the System Information message, for example, to transmit the information.
In TDMA systems, information is usually transmitted in time slots of a TDMA frame at a certain operating frequency. In order for the bursts transmitted by a mobile station to be timed correctly for the right time slot the transmission moment of the mobile station must be determined also on the basis of the distance between the mobile station and base station in such a manner that the propagation delay of the burst provides for a timing advance for the mobile station""s transmission time. For example, in the GSM the maximum timing advance is 233 xcexcs, corresponding to a 35-km propagation delay. The mobile station""s timing advance parameter may take values from 0 to 63, where 63 corresponds to the maximum timing advance 233 xcexcs.
FIG. 1 shows in the form of a flow diagram a procedure according to the GSM for establishing a connection with a base station. The procedure may start e.g. when a call directed to a given mobile station arrives at the switching center MSC of the traffic area in which the called subscriber is located at that moment. The switching center transmits a paging request to one or more base stations in that particular traffic area, which base stations are those to whose coverage areas the called subscriber is registered. The base stations transmit a paging message onto the radio path through their paging channels PCH, phase 10. A mobile station that notices a subscriber identifier belonging to it on the paging channel that it monitors will transmit a CHANNEL REQUEST message to the base station, 12. If the user of the mobile station wants to establish a connection, it is initiated by transmitting to the base station a corresponding CHANNEL REQUEST message, 12. This message is transmitted in a random access type burst on a RACH channel. Having received the CHANNEL REQUEST message the base station determines a timing advance value for the mobile station and transmits it and other information concerning the connection to the mobile station in an IMMEDIATE ASSIGNMENT message, 14. The mobile station then starts using the traffic channel and timing advance value indicated by the base station, 16.
FIG. 2 is a flow diagram depicting a procedure according to the GSM for making handover between the serving base stations. First, the current serving base station sends a HANDOVER COMMAND message to the mobile station, phase 20. This message indicates the new serving base station. Then the mobile station sends to the new base station HANDOVER ACCESS bursts, phase 22. These random access type bursts are transmitted on a Dedicated Control CHannel (DCCH). Having received the HANDOVER ACCESS message the new base station determines a timing advance value for the mobile station and transmits it and other related information to the mobile station in a PHYSICAL INFORMATION message, phase 24. If the handover is synchronous, however, the mobile station already knows the timing advance. The mobile station then starts using the traffic channel and timing advance value indicated by the base station, phase 26. Finally, the mobile station transmits to the new serving base station a HANDOVER COMPLETE message, phase 28, whereafter the old base station no longer serves the mobile station.
In some cases it has become necessary in TDMA systems to introduce so-called extended cells in which the operating range could be greater than the normal range 35 km (for example). One reason for this need is the introduction of lower frequencies. For instance, a 450-MHz GSM system could operate at a distance of up to 120 km from base station as far as the allowable transmission power is concerned because signal attenuation in relation to the distance is smaller than at higher operating frequencies. However, problems are then caused by the fact that the maximum timing advance is not sufficient to compensate for the propagation delay of the signal.
FIG. 3 shows two base stations BTS1301 and BTS2311 of a cellular system according to the prior art and their operating ranges. Cells 302 and 312 of the base stations are conventional cells in which the maximum operating range is 30 km from the base station. At that distance the timing advance is 54. Cells 303 and 313 of the base stations are conventional cells, too, with a maximum operating range of 35 km from the base station, in which case the timing advance is 63. As said, this timing advance is the maximum timing advance in a conventional GSM system. Cells 304 and 314 are extended cells in which the maximum operating range from the base station is 120 km. The maximum timing advance parameter required in this case is 215.
FIG. 3 also shows a mobile station MS 320. When the mobile station is moving in the direction of the arrow it is first connected with base station BTS1 and, after a while, it has to make handover from BTS1 to BTS2. If the system allows for timing advance parameters greater than 63, the problem is how to transmit an access request from the mobile station to base station in such a manner that the base station is able to receive it. Normally the mobile station sends the random access request with a timing advance parameter value 0. If, however, the correct timing advance parameter value were greater than 63, the random access request would not arrive at the base station within the time window reserved for the message.
FIG. 4 illustrates a situation in which a mobile station becomes connected to a base station/makes handover serving base stations. The figure shows a TDMA frame in a possible current cell 1, 410, comprising time slots 411 to 418. Time slot 411 contains a control channel CCCH. The potential traffic channel of the mobile station MS is located in time slot 415 in this cell. Let us consider a situation in which the mobile station establishes a connection with cell 2, 420. In this case the extended cell comprises TDMA time slots 421 to 428. The mobile station transmits a CHANNEL REQUEST or HANDOVER ACCESS burst to the control channel of cell 2. This is depicted by arrow 431. When the mobile station after that learns the correct timing advance of cell 2, it can transmit on the traffic channel TCH allotted to it, located in this case in time slot 425. This is depicted by arrow 432.
In the case of an extended cell, the second time slot 422 of the TDMA frame cannot be used as a traffic channel because the HANDOVER ACCESS burst uses the time slot 422 if the mobile station is located farther than the operating range of a normal cell. In that case the base station must be able to receive the ACCESS burst also at time slot 422. If the CCCH time slot is followed by a time slot for receiving the ACCESS BURST, it is possible to operate at a distance of a propagation delay of 580 xcexcs. Then it would be possible to use a maximum value of 219 for the timing advance parameter in cells that support the extended operating range. The value 219 is obtained by adding to the normal maximum value 63 the length of one time slot in bits, i.e. 156. However, a disadvantage of this solution is that it reduces the data transmission capacity reserved for the traffic channels.
The object of the invention is to provide a solution for establishing a connection between a mobile station and base station, by means of which the aforesaid disadvantages associated with the solutions of the prior art can be avoided.
An idea of the invention is that when establishing a connection a mobile station transmits access bursts using a preferably extended timing advance different from zero. The mobile station preferably uses different timing advances until with a certain timing advance the burst hits a CCCH channel time slot in the base station. The mobile station preferably transmits the first bursts using the normal timing advance, the next bursts using a second timing advance, the corresponding timing advance parameter value being the normal timing advance parameter maximum value+1, and the subsequent bursts using a timing advance of 2xc3x97 the second timing advance, etc. This goes on until a response message arrives from the base station or until the mobile station has gone through the whole timing advance range available.
The invention eliminates the need to reserve an extra time slot off traffic channel use, and the traffic capacity can thus be increased. In addition, since there is no need to monitor CHANNEL REQUEST and HANDOVER ACCESS bursts during two successive time slots, there is less need for changes in the base station""s digital signal processing algorithms in conjunction with the introduction of the extended operating range. By means of the solution a conventional system can thus simply be turned into a system utilizing extended cells.
The method according to the invention for establishing a connection between a mobile station and base station, where extended timing advance is used in actual data transmission and where the mobile station transmits an access burst to the base station, is characterized in that the access burst is transmitted using a timing advance value that differs from zero.
A cellular system according to the invention which comprises means for changing the serving base station in a time-division cellular network, where extended timing advance is used in the actual data transmission between the base station and mobile station and where the system comprises means for receiving an access burst from the mobile station, is characterized in that the base station comprises means for receiving an access burst that has a timing advance value different from zero.
A mobile station according to the invention which comprises means for establishing a connection between the mobile station and base station in a cellular network, where extended timing advance is used in the actual data transmission between the base station and mobile station and the mobile station comprises means for transmitting an access burst to the base station, is characterized in that the mobile station comprises means for transmitting an access burst that has a timing advance value different from zero.
Preferred embodiments of the invention are presented in the dependent claims.
Extended timing advance means in this patent application that the maximum value of the timing advance used corresponds to the propagation time delay which causes the access burst to shift outside the normal time slot window.